WASHINGTON (Reuters) - Thousands of U.S.-backed fighters in Syria are launching an offensive to capture from Islamic State a crucial swathe of northern Syria known as the Manbij pocket following weeks of quiet preparations, U.S. officials disclosed to Reuters.

The operation, which only just started to get underway on Tuesday and could take weeks to complete, aims to choke off Islamic State's access to Syrian territory along the Turkish border that militants have long used as a logistics base for moving foreign fighters back and forth to Europe.

"It's significant in that it's their last remaining funnel" to Europe, a U.S. military official said.

A small number of U.S. special operations forces will support the offensive on the ground, acting as advisors and staying some distance back from the front lines, the officials said, speaking on condition of anonymity to discuss military planning.

"They'll be as close as they need to be for the (Syrian fighters) to complete the operation. But they will not engage in direct combat," the official said.

The operation will also count on support from U.S.-led coalition air strikes as well as from ground-based firing positions across the border in Turkey.

Perhaps essential for NATO ally Turkey, the operation will be overwhelmingly comprised of Syrian Arabs instead of forces with the Kurdish YPG militia, who will only represent about a fifth or a sixth of the overall force, the officials said.

Ankara considers the Syrian Kurdish YPG fighters to be terrorists and has been enraged by U.S. backing for the militia in its battle with Islamic State in Syria.

YPG TO WITHDRAW

Turkey has been alarmed by advances by Kurdish forces along its border and opposed the idea of YPG fighters taking control of the Manbij pocket. The Kurdish YPG militia already controls an uninterrupted 400 km (250 mile) stretch the border.

The officials told Reuters, however, the YPG will only fight to help clear Islamic State from the area around Manbij. Syrian Arab fighters would be the ones to stabilize and secure it once Islamic State is gone, according to the operational plans.

"After they take Manbij, the agreement is the YPG will not be staying ... So you'll have Syrian Arabs occupying traditional Syrian Arab land," the official said, adding Turkey supported the offensive.

The operation comes ahead of an eventual push by the U.S.-backed Syrian forces toward the city of Raqqa, the Islamic State's defacto capital in Syria and the prime objective in Syria for U.S. military planners.

The U.S. military official said depriving Islamic State of the Manbij pocket would help further isolate the militants and further undermine their ability to funnel supplies to Raqqa.

U.S. President Barack Obama has authorized about 300 U.S. special operations forces to operate on the ground from secret locations inside Syria to help coordinate with local forces to battle Islamic State there.

In a reminder of the risks, one U.S. servicemember was injured north of Raqqa over the weekend, the Pentagon said.